In the production of aluminum by electrolysis in a molten bath of Al.sub.2 O.sub.3 being dissolved in molten cryolite, a crust is formed on the surface of the electrolytic bath. This crust must be broken and pushed down into the molten electrolyte in order to promote the process of electrolysis.
The present invention relates to a crust-breaking machine for the above purpose, this machine being of the type which comprises a self-propelled carriage having front and rear wheels, a crust-breaking tool of the continuously breaking type mounted on the carriage, and an arm provided on the carriage and adapted to carry the tool. The term crust-breaking tool of the continuously breaking type is herein intended to refer to either a freely rotatable tool in the form of a wheel having a set of radially projecting rods, or a plough-like tool. Both these embodiments of a continuously breaking tool are described in U.S. Pat. No. 3,306,668.
As is known, common electrolytic cells are usually rectangular. The cells are arranged either end to end or side by side, and usually several cells are arranged in rows inside a furnace hall. Between each row there is a driving lane. The crust on the electrolytic bath is broken either at substantially fixed points of time, for example 5 - 15 times a day ("round breaking") or when the cell voltage has increased because of lack of Al.sub.2 O.sub.3 in the electrolyte ("breaking upon anode effect").
When the cells are placed end to end the crust is usually broken at the longitudinal side of the cell, but when the cells are placed side by side, the crust may be broken either at the longitudinal side or at the short side. In order to secure a smooth operation the crust is usually broken every second time at one side and every second time at the other side of the cell. When the crust is broken as "round breaking" the carriage usually starts at one end of a row and breaks the crust on most of the cells while driving towards the other end of the row. When the crust is broken "upon anode effect", this usually takes place at a point of time which is not determined beforehand, and an anode effect condition can occur at one end of the row whereas the following anode effect condition can occur at the other end of the same row, or it may occur on another row.
In the carriages employing a crust-breaking tool as described in U.S. Pat. No. 3,306,668 the arm assembly can only be swung in a vertical plane transversally to the longitudinal direction of the carriage. In this way the crust-breaking tool can only work at one side of the carriage.
When the crust is broken as "round breaking" it is desired for safety reasons that the carriage shall always have the same driving direction, i.e. it shall always be driven either forwardly or backwardly. With the arm arrangements having existed hitherto, this results therein that when it is desired to break the crust at one side of the cell, one starts at one end of the row, whereas when it is desired to break the crust at the other side of the cell, one has to start at the other end of the row. In this way the crust on the cells being at the middle of the row can be broken at a fixed point of time, whereas on the cells being located at the ends of the row, the crust will be broken either a little earlier or a little later than desired.